Ueno Zoo. Ueno Park Area.

I didn't think this zoo was anything spectacular. Mostly it was really depressing. However, the area around the zoo is super awesome! So go hang out there instead!

This zoo, nestled in the middle of Ueno Park, provides a great escape from the streets of Tokyo without requiring you to leave the city. Even better, you can see all the animals you want for free if you go on March 20 (Ueno Zoo's Anniversary Day), May 4 (Greenery Day), and October 1 (Tokyo Citizens' Day). [Update: Totally understand Globetrotter's perspective. I live here and was randomly hanging out in the park with my friends on what happened to be a free admission day. We were just so happy to see so many animals for nothing! If the date is right and you have the chance, I stand by my recommendation.]